In a typical electrostatographic machine using, for example, an electrophotographic process, an image is generated by first charging a photoconductive element and generating an electrostatic latent image by image-wise exposing the electrostatic latent image using either an optical or an electro-optical exposure system, such as a laser scanner or LED array. The electrostatic latent image is then developed into a visible image by bringing the latent image bearing photoconductive element into contact with an appropriate developer. Most commonly, so-called "two-component developers", comprising electrostatically charged marking or toner particles and oppositely charged nonmarking or carrier particles, are used, although monocomponent developers are also used. The image is transferred to a receiver sheet such as paper or transparency stock using suitable means such as by applying an electrostatic field so as to urge the toner particles from the photoconductive member to the receiver sheet. The image is then permanently fixed to the receiver sheet by a known suitable process such as fusing, while the photoconductive element is cleaned and made ready for reuse. Color images are generally made by producing latent images corresponding to separations of the primary colors (e.g. cyan, magenta, yellow, and black) and transferring them sequentially, in register, to the receiver sheet. Alternatively, it is known to form two-color images in an image frame of a photoconductive element and transfer the images simultaneously to a receiver sheet; see for example, Gundlach U.S. Pat. No. 4,078,929.
It is often advantageous to transfer toner images initially formed on the photoconductive element or other primary toner image forming member to an intermediate member first and subsequently transfer the toner images to the receiver sheet. In this instance, color images (including so-called "spot-color" images) are prepared generally by transferring all separation toner images sequentially, in register, to the intermediate member and, subsequently, transferring the image from the intermediate member to the receiver sheet. However, under some circumstances, it may be desirable to transfer separations to the intermediate or separate intermediates, and register the separations during the transfer to the receiver sheet.
Of particular interest is the use of electrophotographic apparatus comprising a compliant transfer intermediate, as described in the patents by Zaretsky, U.S. Pat. No. 5,187,256, and Rimai et al, U.S. Pat. No. 5,084,735. These intermediates typically comprise an electrically conducting cylindrical core, such as aluminum, overcoated with a semi-insulating elastomeric blanket. Further, these intermediates have a thin relatively hard overcoat of thickness no more than about 30 .mu.m comprised of a material which serves to control particle adhesion. Such a structure has been found to be beneficial in reducing transfer generated image artifacts such as hollow character (the failure to transfer the centers of fine lines) and halo (the failure to transfer toned regions adjacent to high density areas). In addition the use of compliant transfer intermediates has been shown to be of value when transferring images comprised of small size toner particles (i.e. mean volume weighted diameters between about 2 .mu.m and about 9 .mu.m). Mean volume weighted diameter of toner particles may be measured by conventional diameter measuring devices such as a Coulter Multisizer, sold by Coulter, Inc. Mean volume weighted diameter is the sum of the mass of each particle times the diameter of a spherical particle of equal mass and density, divided by total particle mass.
In the preferred embodiments described herein, transfer of toner images is made from a toner bearing compliant image member (TBCIM) to a receiver sheet or member such as paper or plastic transparency. For the purpose of this specification, a TBCIM is defined as an image bearing member comprising a polymeric layer not less than 1 mm in thickness, said layer having a Young's modulus between 0.5 MPa and 50 MPa. (MPa are mega Pascals or 10.sup.6 Newtons/meter.sup.2). Polymers having such properties are frequently referred to as elastomers and, for the purpose of this specification, elastomers are defined as such. A TBCIM can comprise a primary image forming member such as a photoconductor such as that disclosed by Tombs and May in U.S. application Ser. No. 08/655,787. Alternatively, it can comprise a compliant intermediate transfer member such as those disclosed by Rimai and Zaretsky referred to above. Preferred TBCIMs are described in U.S. application Ser. No. 08/846,056, filed in the name of Vreeland et al. Transfer, in general, occurs in the nip region formed between the TBCIM and a backup roller or other pressure application device located behind the receiver sheet.
A well known property of elastomers such as those used in a TBCIM is often described by the principle of time-temperature superposition. According to this principle, a polymer, which behaves as an elastomer under normal conditions, appears quite rigid or glassy under conditions where it is subjected to a sudden force or impact by another object. Specifically, under impact conditions, a typical elastomer would behave like a material having a Young's modulus of approximately 3 GPa. Such an impact is often caused by a receiver sheet entering a transfer nip region. A material with that effective magnitude of Young's modulus would not be able to rapidly conform to the receiver. Because of the mass of the intermediate, this impact can jar or momentarily stall the machine, thereby adversely affecting image quality. In particular it can cause the separations to be misregistered during the transfer process. Moreover, in some applications, such impacts can adversely affect the timing of the engine, thereby creating user-observable artifacts in the final image, as well as causing general wear and tear to the engine.
Although the previous discussion relates principally to a drum intermediate, it can also apply to a web intermediate, especially if additional pressure is applied to the web intermediate member during transfer by use of a back-up roller. Similarly, it can also apply to a web or drum compliant primary image member.
There are many receivers used in electrophotographic engines. These receivers may have vastly different physical properties. One such property is the thickness of the receiver member or receiver sheet which generally varies from less than 75 .mu.m to well over 250 .mu.m. The use of an intermediate transfer member with a single imaging member eliminates the need to wrap the receiver around a drum to produce color images and, thereby, facilitates the use of even much thicker stock (over 0.5 cm thick). Receiver sheet thickness can vary between jobs or even within a job, as would be the case where booklets with covers are being prepared. A thick receiver sheet can create a substantial impact upon entering the transfer nip, as previously discussed.
Not only can undesirable impulse shocks adversely affect output image quality, but they can also adversely affect the reliability and life of a transfer station, including the TBCIM.
There is a need to minimize mechanical disturbances caused by receiver members in a transfer station, e.g., in a production machine in which papers of different thicknesses are needed for different job streams, or when receiver member thickness changes abruptly within a particular job stream.